1. Field of the Invention
This invention relates to electronic circuitry technology, and more particularly, to a peripheral device connection current compensation circuit which is designed for use in conjunction with a peripheral interface controller on a computer platform, such as a USB (Universal Serial Bus) interface controller, and which is capable of being activated when an external peripheral device, such as a USB-compliant portable flash memory unit, is connected to the USB interface controller to provide a current compensation function that can help prevent an electrical current induced by a pull-up resistor in the peripheral device from flowing into the internal circuitry of the peripheral device, such that the slew rate of the USB connecting circuitry can be maintained at the rated value.
2. Description of Related Art
The computer industry has developed a portable thumb-size data storage unit, such as USB (Universal Serial Bus) flash memory, USB hard disks, etc., which allow the user to conveniently transfer data from one PC (personal computer) to another. Presently, most portable thumb-size data storage units are compliant with the standard USB or FireWire interface on PCs such as desktop computers, notebook computers, tablet computers, network workstations, and so on. These portable thumb-size data storage units are characterized by a plug-and-play (PnP) capability that allows instant use without the need to apply electrical power and install drivers on the PC. Since these thumb-size data storage units are very small in size and thus highly portable, they are becoming a very popular peripheral device on the PC market.
Presently, the USB standard includes two subtypes: USB1.0 and USB1.1. To allow a PC to distinguish whether an externally connected peripheral device is USB1.0 or USB1.1 compliant, a widely used method is to connect a pull-up resistor inside the peripheral device. As shown in FIG. 1, by the USB specification, a standard USB-compliant peripheral device includes four connecting lines: a power line (Vbus) 11, a minus data line (D−) 12, a plus data line (D+) 13, and a grounding line (GND) 14. In the case that a portable USB flash memory unit 20 is USB1.0 compliant, a pull-up resistor 22 is interconnected between the power line (Vbus) 11 and the minus data line (D−) 12 as illustrated in FIG. 1; whereas in the case of USB1.1, a pull-up resistor (not shown) will be interconnected between the power line (Vbus) 11 and the plus data line (D+) 13. When the USB1.0 compliant flash memory unit 20 is connected to a PC (not shown), the pull-up resistor 22 will cause the minus data line (D−) 12 to be set at logic-HIGH voltage state, thereby allowing the PC's USB peripheral controller (not shown) to recognize that the externally connected flash memory unit 20 is USB1.0 compliant and thereupon perform a corresponding configuration procedure for the flash memory unit 20.
One problem to the connection of the above-mentioned pull-up resistor 22 in USB1.0 peripheral device, however, is that it would undesirably cause an induced electrical current to flow from the power line (Vbus) 11 to the minus data line (D−) 12 and then into the internal circuitry 21 of the USB flash memory unit 20. This phenomenon would cause the slew rate of the USB connecting circuitry to exceed its rated value, thus undesirably causing instability in data transmission to and from the USB flash memory unit 20.